1. Field of the Invention
The invention relates in general to the field of polychromatic printing and relates to a method and apparatus for avoiding image defects which arise due to faulty registration caused by inaccurate superposition during printing of the color separations of the individual printing inks magenta (M), cyan (C), yellow (Y) and black (K) wherein an original image master is opto-electronically scanned and trichromatic primary measured color value signals red, green and blue are obtained during scanning and are converted into color separation signals for the printing inks magenta, cyan, yellow and black and the color separation signal control the production of the various color separations which are used for printing and wherein at the edges of the zones between one area of one color and another one color is displaced under the edge of the adjacent different color at the color separation boundaries so as to eliminate misalignment and inaccuracies of the prior art.
2. Description of the Prior Art
Present day standard polychromatic printing where subtractive color mixing is used, "flashers" which have a disruptive effect and reduce the image quality at the edges of color and also white passages occur wherein the white of the paper appears due to misregistration of the superimposed color separations of the colors in the printers and due to other causes as, for example, extreme color removal in the individual color separations are due to inaccurate registration during copying of the images.
A German OS No. 24 46 761 discloses a method for additive chromatic printing wherein dark color areas are reduced by a prescribed width and light color areas are expanded in the direction over the dark color areas so as to compensate for bleeding of the inks and to assure the meeting of the color areas during actual printing. With this type of printing, a color separation is produced for each individual printing ink color and depending upon the desired color placement, the color separations are inked with an ink having a prescribed color composition, for example, pink, violet or brown, etc. Based on the master, thus, the desired color pattern occurs by means of superimposed printing of the color separations even in the colors of the original master according to a selected color setting in another color pattern which is different from the original. Colors having a precisely fixed color composition are employed both in producing the original master as well as during printing so that on the one hand the colors can be exactly observed during the opto-electronic scanning of the original masters and on the other hand, so that the corresponding image portions or image pattern elements appear in the desired colors in the printing process.
This printing method makes it simple to avoid flashes appearing due to registration, misalignment or misregistration, color margins or, respectively, undesired overlaps or non-uniform widths of lines in dark color boundaries because an enlargement or reduction of the surfaces of the individual color areas is done in that dark color areas are reduced at all locations by a prescribed width and light areas are enlarged in the direction of and over a dark color areas. The criteria for accomplishing this method is simple since only two colors of a precisely defined color composition meet in the image portions which are being considered and when selected color settings are used it is easy to distinguish or, respectively, determine between the two colors which forms the lighter or, respectively, the darker areas. Also, the overall number of colors employed is limited to a low number such that a clear statement concerning this criteria can be obtained at any time.
When subtractive color mixing is utilized in polychromatic printing and recognition of individual colors does not proceed from the pattern master completely different conditions exist. A plurality of different colors can appear next to one another at image passages so that errors of the types described above can occur. This can be visualized from the fact that the individual printing inks can assume all intermediate values between 0 and 100% depending upon the color of the original master and that all colors of the color space can occur. Thus, unlike the method described in German OS No. 24 46 761 which of two colors at a contour is the lighter or, respectively, the darker is not identified but rather a determination must be carried out as to which of the color signals obtained during scanning of the original master is the brighter or darker printing ink.